This invention relates generally to processes and apparatus for the removal of nitrogen oxides or “NOx” (principally nitric oxide (NO) and nitrogen dioxide (NO2)) from exhaust gases and the like. More particularly, the present invention relates to processes and apparatus for reducing NOx selectively from exhaust gases produced during petroleum refining, petrochemical production and also to industrial processes producing exhaust gases containing NOx.
Carbonaceous fuels are burned in internal combustion engines and in a wide variety of industrial process (i.e. boilers, furnaces, heaters and incinerators, petroleum refining, petrochemical production, and the like). Excess air frequently is used to complete the oxidation of combustion byproducts such as carbon monoxide (CO), hydrocarbons and soot. Free radicals of nitrogen (N2) and oxygen (O2) combine chemically to form NOx, primarily NO, at high combustion temperatures. This thermal NOx tends to form even when nitrogen is not present in the fuel. Combustion modifications which decrease the formation of thermal NOx generally are limited by the generation of objectionable byproducts or deteriorating flame properties.
When discharged to the air, NO emissions oxidize to form NO2, which in the presence of sunlight reacts with volatile organic compounds to form ground level ozone, eye irritants and photochemical smog. Despite advancements in fuel and combustion technology, ground level ozone concentrations still exceed federal guidelines in many urban regions. Under the Clean Air Act and its amendments, these ozone non-attainment areas must implement stringent NOx emissions regulations. Such regulations require low NOx emissions levels that are attained only by exhaust after-treatment. When an exhaust after-treatment system is applied to a refinery or petrochemical plant, it is particularly important to minimize any impact on the operation of the underlying refining or petrochemical process.
Exhaust after-treatment techniques tend to reduce NOx using various chemical or catalytic methods. Such methods are known in the art and involve non-selective catalytic reduction (NSCR), selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Alternatively, NO may be oxidized to NO2 for removal by wet scrubbers. Such after-treatment methods typically require some type of reactant for removal of NOx emissions.
Wet scrubbing of NO2 produces waste solutions that represent potential sources of water pollution. Wet scrubbers primarily are used for NOx emissions from nitric acid plants or for concurrent removal of NO2 with sulfur dioxide (SO2). High costs and complexity generally limit scrubber technology to such special applications.
The NSCR method typically uses unburned hydrocarbons and CO to reduce NOx emissions in the absence of O2. Fuel/air ratios must be controlled carefully to ensure low excess O2. Both reduction and oxidation catalysts are needed to remove emissions of CO and hydrocarbons while also reducing NOx. The cost of removing excess O2 precludes practical applications of NSCR methods to many O2-containing exhaust gases.
Chemical reactions on a solid catalyst surface of commercial SCR systems convert NOx to N2. These solid catalysts are selective for NOx removal and do not reduce emissions of CO and unburned hydrocarbons. Large catalyst volumes are normally needed to produce low levels of NOx. The catalyst activity depends on temperature and declines with use. Normal variations in catalyst activity are accommodated only by enlarging the volume of catalyst or limiting the range of combustion operation. Catalysts may require replacement prematurely due to sintering or poisoning when exposed to high levels of temperature or exhaust contaminants.
Commercial SCR systems primarily use ammonia (NH3) as the reductant. Excess NH3 needed to achieve low NOx levels tends to result in NH3 breakthrough as a byproduct emission. Even under normal operating conditions, SCR systems require a uniform distribution of NH3 relative to NOx in the exhaust gas. NOx emissions, however, are frequently distributed nonuniformly, so low levels of both NOx and NH3 breakthrough may be achieved only by controlling the distribution of injected NH3 or mixing the exhaust to a uniform NOx level.